Why does C 11's standard layout definition impose restrictions on access controls and data member ordering?

Standard Layout in C 11: A Decomposition for Efficient Data Representation

In C 11, a revised definition of Plain Old Data (POD) classes stipulates a set of criteria known as "standard layout." This definition has specific constraints that may raise questions regarding its rationale.

Varying Access Controls for Data Members

The prohibition against varying access controls for non-static data members ensures uniformity of memory layout. If different access levels were permitted, it would create ambiguity in accessing object members, particularly when casting a pointer to the first member.

Data Members as Base Classes

Prohibiting the first data member from being the same type as a base class prevents address conflicts. C does not specify the allocation order of data members across classes. If the first data member were the base class, the compiler could not determine the "first" member for casting purposes.

Multiple Data Members in Constituent Classes

Restricting data members to at most one base class or derived class ensures predictable memory allocation. If both the derived class and base class had data members, the Standard's lack of defined allocation order would make it impossible to determine the first data member's address for casting.

Implications for Memory Allocation

The standard layout definition is designed to facilitate efficient memory allocation. The same address can be used to represent the first data member and the complete object, allowing for efficient casting of object addresses. By prohibiting varying access controls and member ordering conflicts, the Standard ensures consistent memory layouts that optimize data access and manipulation.

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